While this invention can be used on injection molds and pre-blow molds, it is intended primarily for use with the mold into which air is injected through a core rod to expand the plastic parison on the core rod to the shape of the mold cavity into which the core rod extends. The plastic is stretched both lengthwise of the core rods and circumferentially about the axis of the core rods, and if this stretching occurs within the proper temperature range, the plastic is bi-axially oriented with resulting increase in the strength of the final product made from the plastic.
The fact that the parison is stretched in the blow mold and the amount of stretch for different length bottles may vary and also that the amount of stretch may vary with material makes it desirable to vary the position of the center of pressure.
Blow molds are generally made with a fixed portion and a movable portion that reciprocates toward and from the fixed portion for each blowing operation. The blow cavity is generally formed partly in the fixed portion and partly in the movable portion of the mold, and the molds fit together so that there is little or no indication on the molded product at the location where the mold portions came together. Motor-operated means shift the movable mold portion toward and from the fixed mold portion; and it is essential that the two portions of the mold fit together accurately.
For a number of reasons, it is advantageous to have the force that holds the mold closed applied in line with the center of pressure of the air or other fluid in the mold cavity. If a blow molding machine could be designed for making only a single product, always of the same size, there would be no problem. However, blow molding machines are expensive apparatus and customers do not want machines that are limited to the making of only a single article.
This invention provides a simple and effective construction by which a blow molding machine can be used with molds of different sizes and with mold cavities of different sizes and still maintain the force holding the mold closed in line with the center of pressure of the fluid which is injected into the cavity. The invention will be described as applied to a mold having a single cavity; but no matter how many cavities the mold has, there is always a center of pressure of the fluid which is injected into the mold.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.